: I don't know why the excrement of bulls was selected over that of any other animal for this particular honour.

I've heard it was the excrement of choice for this expression because bulls, being stronger than cows, were used as draft animals on many farms; therefore the hapless farmer was frequently stepping in the substance.
Coming from a rural area I've heard horseshit and pigshit used almost as often. About thirty years ago whaleshit was a popular phrase; being on the bottom of the ocean, it was lower than any other kind.